1. Field of the Invention
The present invention relates to a PCM (Pulse Code Modulation) recording and reproducing apparatus in which an analog audio signal is converted to a digital signal and recorded in the form of digital signal and a reproduced digital signal is converted to the original analog audio-signal, and more particularly to a PCM recording and reproducing apparatus capable of distinguishing digital signals of different texts, that is, a digital signal which is a digitized version of the original audio information and a digital signal which is a digitized version of system control information for the PCM recording and reproducing apparatus, without adding redundancy and with a simple hardware.
2. Description of the Prior Art
Apparatus for converting an analog audio signal to a digital signal for recording has been well known as a PCM recorder. Information related to the analog audio signal to be recorded by the PCM recorder includes at least the following three information.
The first information is a coded word which represents amplitudes of the analog audio signal at predetermined time points by "1" or "0" binary digital representation. (Hereinafter, it is referred to as a sampled signal word.)
The second information is a frame synchronizing signal to specify a leading position of the sampled signal word block when a predetermined number of sampled signal words are assembled into a block called frame (hereinafter referred to as a data frame). It is inserted at the beginning or the end of the block.
The third information is an error detection code for detecting and handling a code error when it occurs in the sampled signal word due to the dropout which occurs during the recording and reproducing of the PCM recorder. The error detection code is needed because the error code is recognized as a noise as it has no connection with the original audio signal.
In the error detection of the digital signal, the word-by-word parity checking is usually used. Recently, the cyclic redundancy check code (hereinafter referred to as CRC) having a high probability of error detection has been widely used.
The information to be recorded in the PCM recorder basically includes three information described above, that is, the sampled signal words which are digital version of the analog audio signal, the frame synchronizing signals and the error detection code such as CRC. Practically, a coded control signal is additionally required. The control signal is not directly related to the audio information but it is included for the purpose of system control to render the PCM recording and reproducing apparatus easier to use. Examples of the control signal are address signals inserted at a predetermined interval to render the edition easier or to indicate the beginning of music, identification code for identifying the presence or absence of preemphasis, and identification code for the recorded texts.
Heretofore, the control signal was recorded for each data frame, or when a video signal is recorded in the PCM recorder, the control signals were inserted in group for each vertical synchronizing signal of a television composition synchronizing signal. Several tens bits or more are necessary to record the control signals while the number of bits in the frame is in the order of 100 bits for the PCM recording. Accordingly, the control signals may need a space comparable to one frame. In addition, the control signals need not be inserted too frequently from the purpose thereof and the lower frequency of insertion thereof is rather desirable because a necessary band width for recording can be reduced. When the control signal frame is inserted for every plurality of data frames, it is necessary to distinguish the data frame from the control signal frame. Like the data frame, the control signal frame includes the frame synchronizing signal and the error correction code such as CRC.
Thus, in the PCM recorder, the control signal is inserted in one horizontal scan section immediately following to the vertical synchronizing signal of the composite synchronizing signal. In a storage device of a computer, the control signal is recorded at a preassigned position relative to a reference position. In the PCM recorder which does not record the vido signal, however, it is necessary to specify the beginning of data (BOD) and the end of data (EOD) as is the case of the compuer storage device. This is not desirable because it results in the increase of the reducancy in the recording and complex processing by hardware because of the frequent need for the determination of the EOD and the BOD.